Input output concepts in operating systems

P.C.P Bhatt OS/M5/V1/2004 1
Input Output (IO) Management
Prof. P.C.P. Bhatt

• Humans interact with machines by providing
information through IO devices.
• Many on-line services are availed through use of
specialized devices like printers, keyboards etc.
• Management of these devices can affect the throughput
of a system.
Introduction

Communication with an IO device is required at the
following levels:
¾ The need for a human to input information and
output from a computer.
¾ The need for a device to input information and
receive output from a computer.
¾ The need for computers to communicate over
networks.
Issues in IO Management

¾Character oriented input devices operate with
speeds of tens of bytes per second.
¾Block oriented devices are much faster than
character oriented devices. Also note that they
operate over a much wider range.
Devices communicate bit or byte streams with a
machine using a data bus and a device controller.
Device Speeds

• A computer system may have to synchronize with
some other process to communicate.
• So, one process may actually wait at the point of
rendezvous for the other process to arrive.
• The process advances further following the
synchronizing event.
Event Based I/O -1

• Processes may use signals to communicate events.
• Processes may wait for asynchronous events to occur.
• Every OS provides a set of mechanisms which may be
like polling, or a programmed data transfer, or an interrupt
mechanism, or even use DMA with cycle stealing.
Event Based I/O -2

IO Organization -1
Computers employ the four basic modes of IO operation:
These are:
1. Polling
2. Programmed mode
3. Interrupt mode
4. DMA mode

Polling
• Polling: Computer interrogates each device in turn to
determine if it is ready to communicate.
• Polling as a strategy is also used by systems to interrogate
ports on a computer communication network.

9 Programmed mode :
An IO instruction is issued to a device.
Now the program busy-waits (idles) till the device IO
is completed.
In other words execution of I/O instruction is in
sequence with other program instructions.
Programmed mode

9 Interrupt mode :
An IO instruction is issued.
The program advances without suspension till the
device is actually ready to establish an IO, when the
process is suspended.
Interrupt mode

9 DMA mode :
The device requests for an IO for a block data transfer.
The execution is briefly suspended and the starting
address and size of data block are stored in a DMA
controller.
DMA mode

9 Programmed IO is used for synchronizing information
or when speed is not critical, e.g. – in the i386 CPU
based PC architecture, there is a notion of listening to
an IO port.
9 Interrupt transfer is suited for a small amount of critical
information – no more than tens of bytes.
9 DMA is used mostly for block devices.
IO Modes: some observations

Direct Memory Access Mode
of Data Transfer

Network oriented traffic can be handled in DMA mode.
Network traffic usually corresponds to getting
information from a disc file at both ends and network
traffic is bursty.
In all the above modes, OS makes it look as if we are
doing a read or a write operation on a file.
Network oriented traffic

In this mode, an IO instruction is issued and the program
advances without suspension.
Program suspension happens when the device is actually
ready to establish an IO.
An Interrupt Service Routine is executed to achieve
device IO.
Interrupt Mode - 1

Process context is stored to help resume computation
later (from the point of suspension.)
Program resumes execution from where it
was suspended.
Interrupt Mode - 2

Interrupt processing may require the following contexts :
Internal Interrupt :
Source of interrupt is a memory resident process or an
event within a processor (due to divide by zero or attempt
to execute an illegal instruction).
Some times malfunctions caused by events
like divide by zero are called traps.
Timer interrupts may also occur as in RTOSs.
Interrupt Types -1

¾ External Interrupts :
Source of interrupt is not internal i.e. other than a process
or processor related event.
Can be caused by a device seeking attention of a processor.
IO device interrupting a program that had sought IO
(mentioned earlier) is an external interrupt.
Interrupt Types -2

¾ Software Interrupt :
Most OSs offer two modes of operation – user mode and
system mode.
A system call made by a user program will require a
transition from user mode to system mode – this
generates a software interrupt.
Interrupt Types -3

Interrupt Line
IRQ
Recognized
End of
Instruction
Interrupt Recognition
Hardware Support to Recognize Interrupt

Let us see how an interrupt is serviced :
Suppose program P is executing an instruction i when
an interrupt is raised.
Assume also an interrupt service routine ISR to be
initiated to service the interrupt.
Interrupt Servicing -1

The following steps describe how the interrupt service
may happen :
1. Suspend the current program P after executing
instruction i.
2. Store address of instruction i+1 in P as return address
for P – PADDRi+1 which is the incremented program
counter value.
This may be stored (RESTORE) in some specific
location or a systems’ data structure or in the code
area of ISR itself.

3. A branch unconditionally to interrupt service
instructions in ISR happens.
4. Typically, the last instruction in the service routine ISR
executes a branch indirect from RESTORE. This
restores the program counter a branch indirect
instruction from PADDRi+1
Thus the suspended program P obtains control of the
processor again.

We will see how source of interrupt may be identified in
the context of different I/O mechanisms like:
Polling :
It interrogates all the devices in sequence
It is slow if there are many devices.
Identification of Source of Interrupts -1

Interrupt Line
CPU
INT Address
----------
--------
Daisy Chain

Pointer to
Service Routine
Vectored Interrupt

HW/SW Interface

9 Usually, an OS kernel resolves IO commands.
9 Following an issuance of an IO command, the kernel
communicates with individual device drivers; which in
turn communicate with IO devices.
9 The application at the top level communicates only
with the kernel.
I/O and the Kernel -1

9 Each IO request from an application generates the
following:
¾ Naming or identification of the device to
communicate.
¾ Providing device independent data to
communicate.

IIO channel: is a small computer to handle IO from multiple sources
-smoothes out IO traffic.
Applications Devices

9 Kernel IO subsystem arranges for the following:
¾ Identification of the device driver.
¾ Allocation of buffers.
¾ Reporting of errors.
¾ Tracking the device usage.

9 The device driver transfers the kernel IO request to
set device controller with the following information :
• Identify read/write request.
• Set controller registers for data transfer (Data
count = 0; where to locate data)
• Keep track when the data has been transferred
(when fresh data is to be brought in).

Device Driver Operation
The figure below shows the sequence which device drivers
follow to handle interrupts:

Management Buffer -1
Example in the figure shows how at different stages the buffer
sizes may differ.

Buffers are usually set up in the main memory or in caches.
Caches are used to obtain enhanced performance.
Buffers absorb mismatch in the data transfer rates of the
processor or memory on one side and the device on the
other.
Management of Buffers - 2

Assume we are seeking input of data from a device. The
various buffering strategies are as follows:
Single buffer : The device first fills a buffer.
Next the device driver hands in its control to the kernel
to input the data in the buffer.
Once the buffer has been used up, the device fills it up
again for input.

Double buffer : Here there are 2 buffers.
Device driver starts by filling buffer-0 and then hands it
to the kernel to be emptied.
In the meanwhile it starts to fill buffer-1.
The roles are switched when buffer-1 is filled out.

Circular buffer : One can say that double buffer is a circular
queue of size 2.
We can have many buffers in a circular queue.
Kernel accesses filled out buffers in the same order that these are
filled out.
Note that buffer management requires management of a queue
data structure.
One must have pointers to the head and tail of this queue to
determine if it is full or empty.

Buffering Schemes

Spooling in Printers
Consider a printer connected to a machine and several
users wanting to use it.
To avoid print clashes, all the print requests are
SPOOLED and thus the requests are queued.
OS maintains and schedules all print requests.
We can examine print queue status with lpq and lpstat
commands in Unix.
Additional Considerations

Clocks and Management of Time -1
Clocks : CPU has a system clock. OS uses this clock to
provide a variety of system and application based services
such as :
• Maintaining time of day (date command in Unix)
• Scheduling a program run at a specified time during
systems’ operation (cron command)

Clocks and Management of Time -2
•Providing over runs by processes in preemptive scheduling
- important for real-time systems.
• Keeping track of resource utilization or reserving
resource use.
• Performance related measurements (like timing IO,
CPU activity).

Identifying Devices
Addressing a device :
•Most OSs reserve some address space for use as exclusive
addresses for devices (like DMA controllers, timer, serial
ports).
•Each of them have fixed ranges of addresses so that they
communicate with the right ports of data.

Caching -1
•A cache is an intermediate level fast storage.
•Caches are regarded as fast buffers.
•Caching may be between disk and memory or memory and
CPU.
•It helps in overcoming the latency during an instruction
fetch.
•It helps in higher locality of reference when used for data.

Caching -1
•As for main memory to disk caches, one use is in disk
rewrites.
•This technique is used almost always to collect all write
requests over a short period of time and subsequently it is
actually written into disc.
•Caching is always done to enhance the performance of
systems.

IO channels : an IO channel is primarily a small computer
to basically handle IO from multiple sources - smoothes
out IO traffic.
OS and CDE : OS provides several terminal oriented
facilities for operations in a Common Desktop
Environment.
IO kernel provides all screen management functions
within the frame work of a CDE.

Primary memory is volatile and secondary memory is
non-volatile.
Primary memory loses its information when power goes
off; unlike secondary memory.
The most common secondary storage is a disc.
Motivation for Disc Scheduling

Information Storage
Organization on Discs - 1

A disc has several platters each with several rings or tracks.
Rings are divided into sectors where information is actually
stored.
Sequentially related information is organized into cylinders.
Information on different cylinders has to be accessed by
moving the arm – seek latency.
Information Storage
Organization on Discs - 2

Rotational delay is due to the waiting time for a sector in
rotation to come under the read or write head.
The motivation for disc scheduling comes from the need
to keep both the delays to a minimum.
A sector stores a lot of other information in addition to a
block of information
Delays in Information Retrieval -1

Information Storage in Sectors.
Delays in Information Retrieval -1
Note that we require 10% of extra information for a 512 byte
data. Clearly, for larger block sizes this constant over head
becomes less significant. However, larger block sizes would
require larger buffers.

A user communicates with files (program, data, system utilities
etc.) stored on discs. All such communications have the
following components.
¾ The IO is to read from, or write into, a disc.
¾ The starting address for communication in main
¾ memory.
¾The amount of information to be communicated
¾The starting address in disc and current status of the transfer.
Scheduling Disk Operations -1

¾Consider a scenario of one process with one request to access data;
a disc access request leads finally to the cylinder having that data.
¾When multiple requests are pending on a disc, accessing
information in a certain order becomes essential – disc access
scheduling.
For example, if there are 200 tracks on each platter, pending requests
may come in the order – 59, 41, 172, 74, 52, 85, 139, 12, 194 and 87.
A Scenario for Information Retrieval

FCFS Policy : The service is provided strictly in the
sequence in which the requests arrived.
The service would be in the sequence :
59, 41, 172, 74, 52, 85, 139, 12, 194 and 87.
In order to compare the effect of implementing a certain
policy, we analyze the arm movements – captures the
basic disc activity.
Comparison of Policies -1

Disc Scheduling Policies
Assume that the arm is located at cylinder number 100.

Shortest Seek First : Disc access is in the order:
87, 85, 74, 59, 52, 41, 12, 139, 172, 194.
Elevator Algorithm : Assume an initial movement is in a
certain direction. Disc access is in the following order :
139, 172, 194, 87, 85, 74, 59, 41, 12.

Circular Span : This scan policy is done in one
direction and wraps around. The disc access will be in
the following order:
139, 172, 174, 12, 41, 52, 59, 74, 85, 87.
From these graphs we find that FCFS is not a very good
policy; Shortest Seek First and Elevator algorithm seem
to perform well as these have least arm movements.

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